iot/zoap: Fix wrong byte-order when retrieving integer options

When retrieving options that represent an integer, the order of the
bytes being considered was inverted, resulting in invalid values being
returned.

Change-Id: I8ba84f77e3402066632c0ba650939266c87a8ea2
Signed-off-by: Vinicius Costa Gomes <vinicius.gomes@intel.com>